SEOUL, South Korea -- South Korean voters broke the conservatives' four-decade grip on parliament Thursday, bringing to power a liberal party that opposed the president's impeachment and may seek closer ties to North Korea.

The election completes a momentous shift in South Korean politics, where conservative-dominated legislatures have checked its few progressive presidents, including President Roh Moo-hyun.

The Uri Party, which had only 49 seats in the outgoing assembly, seized 152 of 299 seats with 99 percent of the vote counted early this morning, KBS news reported, citing official tallies.

Its main rival, the conservative Grand National Party, took only 121 seats. The party, which traces its roots to South Korea's military dictatorship era, had 137 spots in the previous parliament.

Thursday's win for Uri, which backs Roh, could strengthen his hand as the Constitutional Court decides whether to uphold his March 12 impeachment on charges of incompetence and illegal electioneering.

It was South Korea's first impeachment since its founding in 1948.

"This election means that the political forces that have dominated South Korean politics for 44 years are forced to leave the stage," Uri chief Chung Dong-young said.

The Uri Party and Roh believe economic cooperation with North Korea will encourage the communist country open up and ease tensions over its nuclear weapons programs. The GNP accuses them of coddling a Stalinist regime, and has tried to block government-backed aid packages to the North.

The National Election Commission estimated a 59.9 percent voter turnout, up from the previous election's 57.2 percent.

As results came in, GNP head Park Geun-hye acknowledged her party had "made a lot of mistakes."

"I think this election made all political parties, including our Grand National Party, feel what the people want," she said.

Roh wasn't vying for a seat. But many saw Thursday's vote as a referendum on his rule and said an impressive Uri showing could persuade the Constitutional Court to rule in his favor. The impeachment suspended his powers until the court rules.

Roh, who says he plans to join Uri, had pledged to resign as president if the party fared poorly.

"The election results give much needed stability to the Roh Moo-hyun government, which has been struggling with its lack of parliamentary support," said Euh Soo-young, a political science professor at Seoul's Ewha Women's University. "It gives Roh the means to push for his policies."

Official results were expected late Friday morning.

Uri may use its beefed-up position to push for more economic exchanges with the communist North, said Yu Suk-ryol, a political analyst with the Seoul-based Institute of Foreign Affairs and National Security.

"The present government would like closer ties with North Korea," Yu said. "They may also be emboldened to ask the United States to show greater generosity or latitude toward North Korea."

South Korea has been a chief mover behind offering energy aid to the impoverished country in exchange for a nuclear freeze and eventual dismantlement. The United States says it won't offer concessions until Pyongyang proves it is taking its programs apart.

Vice President Dick Cheney arrived in Seoul just as polls were closing Thursday.

Cheney was expected to meet Prime Minister Goh Kun, the acting president while the court rules on Roh, to discuss the nuclear crisis, the U.S.-South Korean alliance and Seoul's pledge to send 3,600 troops to Iraq.

Despite worsening instability in Iraq, the Uri Party confirmed during the campaign it would support the government's plan to dispatch 3,600 troops to northern Iraq later this year.